It is proposed to label monoamine oxidase A and B at their substrate binding sites by reacting the enzymes with (14C)-phenylhydrazine, which alkylates a thiol group at the substrate site of the B enzyme. The two samples will then be denatured, digested with suitable proteolytic enzymes, and the predominant labeled peptides will be isolated and their amino acid sequences compared. It is hoped that this approach will provide an explanation for the well-known differences in substrate specificity and sensitivity to the drugs clorgyline and deprenyl of the two forms of the enzymes. If the project is successful, it may aid the design of more effective and specific drugs for the treatment of Parkinson's disease than presently available.